A variety of systems are used in borehole geophysical exploration and production operations to determine chemical and physical parameters of materials in the borehole environs. The borehole environs include materials, such as fluids or formation matrix, in the vicinity of a borehole as well as materials, such as fluids, within the borehole. The various systems include, but are not limited to, formation testers and borehole fluid analysis systems conveyed within the borehole. In all of these systems, it is preferred to make all measurements in real-time and within instrumentation in the borehole. However, methods that collect data and fluids for later retrieval and processing are not precluded.
Formation tester systems are used in the oil and gas industry primarily to measure pressure and other reservoir parameters of a formation penetrated by a borehole, and to collect and analyze fluids from the borehole environs to determine major constituents within the fluid. Formation testing systems are also used to measure pressure and other parameters of fluid within the borehole. These parametric measurements are typically combined with in situ or uphole analyses of physical and chemical properties of the formation fluid to evaluate production prospects of reservoirs penetrated by the borehole. By definition, formation fluid refers to any and all fluid including any mixture of fluids.
Regarding formation fluid sampling, it is of prime importance that fluid collected for analysis represents virgin formation fluid with little contamination from fluids used in the borehole drilling operation. Various techniques have been used to minimize sample contamination including the monitoring of fluid pumped through a borehole instrument or borehole “tool” of the formation tester system until one and/or more fluid properties, such as resistivity, cease to change as a function of time. Other techniques use multiple fluid input ports combined with borehole isolation elements such as packers and pad probes to minimize fluid contamination. Regardless of the fluid sampling methodology, accurate and precise measurements of fluid pressure are required to obtain meaningful reservoir production information.
Formation tester tools can be conveyed along the borehole by a variety of means including, but not limited too, a single or multi-conductor wireline, a “slick” line, a drill string, a permanent completion string, or a string of coiled tubing. Formation tester tools may be designed for wireline usage or as part of a drill string. Tool response data and information as well as tool operational data can be transferred to and from the surface of the earth using wireline, coiled tubing and drill string telemetry systems. Alternately, tool response data and information can be stored in memory within the tool for subsequent retrieval at the surface of the earth.
Various methods are used to draw fluid into the formation tester tool for pressure measurements, analysis, sampling, and optionally for subsequent exhausting the fluid into the borehole. One method employs a radially extendable sampling pad that is pressed against the wall of the borehole. A probe port or “snorkel” may or may not be extended from the center of the pad through any borehole mudcake to make contact with formation material. Fluid is drawn into the formation tester tool via a flow line cooperating with the snorkel. Formation pressure is determined from a measure of fluid pressure within the probe flow line. In order to isolate this fluid flow into the probe from fluid flow from the borehole or from the contaminated zone, fluid can be drawn into a guard ring surrounding the snorkel. A more detailed description of the probe and guard ring methodology is presented in U.S. Pat. No. 6,301,959 B1, which is here entered into this disclosure by reference. Using a second method, the formation is isolated from the borehole by one or more packers controlled by the packer section of the formation tester tool. A plurality of packers can be configured axially as “straddle” packers. Fluid is drawn into the formation tester tool via a port flow line cooperating with one or more ports disposed in the wall of the tool between the two straddle packers. Formation pressure is determined from a measure of fluid pressure within the port flow line. Straddle packers and their use are disclosed in U.S. Pat. No. 5,337,821, which is incorporated into this disclosure by reference.